Thermal ink-jet printers operate by rapidly heating a small volume of ink and causing the ink to vaporize into a bubble which ejects a droplet of ink through an orifice nozzle to strike a recording medium, such as a sheet of paper. Typically, a number of orifices are arranged in a pattern upon a printhead. Thus, a properly sequenced ejection of ink from each orifice causes characters or other images to be printed upon the paper as the printhead is moved relative to the paper. In this print method, a major component of print quality depends upon the physical characteristics of the orifices in the printhead. For example, the geometry of the orifice affects the size, shape, trajectory, and speed of the ink drop ejected.
An ideal printhead includes nozzle members having re-entrant orifice nozzle profiles. Affixed to a back surface of the nozzle members is a substrate, which channels liquid ink into a vaporization chamber. Liquid ink within the vaporization chamber is vaporized by the energization of a thin film resistor formed on the surface of the substrate that causes a droplet of ink to be ejected from the orifice nozzle. Preferably, nozzle members are formed of a polymer material or a photoresist material using photolithography, laser ablation or other similar techniques to minimize cost and wafer process capability.
Re-entrant nozzles have many advantages over straight-bore or positive sloped nozzles. A re-entrant nozzle is a negatively sloped hole in an orifice layer. The re-entrant nozzle is a hole tapered to form a smaller channel at the orifice layer exit surface than on the substrate surface. This taper increases the velocity of an ejected ink droplet. In addition, the wider bottom opening in the nozzle allows for a greater alignment tolerance between the nozzle and the thin film resistor without affecting the quality of print. Additionally, a finer ink droplet is ejected, enabling printing that is more precise.
Re-entrant nozzles, in which the nozzle is part of a monolithic structure of polymer material on a substrate, are difficult to manufacture using conventional processes. Re-entrant nozzles have been formed using a laser by changing the angle of nozzle substrate with respect to a masked laser beam during the nozzle forming process. An improvement to this technique is to form the re-entrant nozzles with a laser by rotating and tilting an optical element between the laser and the nozzle substrate. Another re-entrant nozzle manufacturing technique is to use two or more masks for forming a single array of nozzles where each mask has a pattern corresponding to a different nozzle diameter. Still another re-entrant nozzle manufacturing technique is to defocus the laser beam during the orifice forming process.
Photolithography approaches have the opportunity to reduce the manufacturing time and reduce the complexity. Masks using projection printing have an opening corresponding to where a nozzle is formed in a photoresist layer. These masks have been used in the past for forming straight and single-angled re-entrant nozzles by controlling the fluence (joules/cm.sup.2) of laser radiation at the target substrate. Another photolithography process uses a single mask to form re-entrant nozzles in a photoresist layer. The mask used is similar to that of projection printing but the opaque and clear portions are reversed. The tapering performed in this process is due to the opaque portions of the mask causing frustum shaped shadows through the photoresist layer corresponding to where nozzles are to be formed. After developing and etching the photoresist layer, the resulting nozzles have a frustum shape. All of the aforementioned various techniques are only able to create one re-entrant nozzle at a time and thus are considered either time consuming, complicated, or subject to error.
Accordingly, what is needed is a process that can form more than one nozzle, preferably an entire printhead array, in a time efficient and highly reliable method using polymer or polyimide materials with either photolithography or optical ablation technology.